The remarkable predilection shown by certain species of microbes for
certain hosts, and the relative indifference of those parasites to other organisms,
has long been the subject of zealous speculation and inquiry. Great as has
been the advance in our knowledge of the underlying causes of natural immunity
during the last few years, it can not be said that any final explanation is
even yet in sight. It is, indeed, apparent that the ardent prosecution of
studies into the germicidal and antitoxic qualities of immune sera has caused
a temporary suspension of activity in other lines of investigation. The broader
biologic aspects of natural immunity sometimes have been neglected. It may
well happen that studies in plant pathology will be found to throw light on
some of the questions of animal parasitism that have hitherto been shrouded
in obscurity. For some time botanists have known much concerning the various
modes by which parasitic fungi gain access to the interior of the host plant.
Some enter through the stomata, some by piercing the walls of the epidermal
cells or the guard cells of the stomata. Marshall Ward, in his study of the
Bermuda lily disease, discovered the interesting fact that the fungus concerned
(a species of Botrytis) effects its entrance into
the host plant by secreting at the tip of the germ tube an enzyme which softens
the substance of the cell wall. Quite recently Massee has brought out important
facts concerning the influences that determine the attack of a specific host
by a specific fungus. It is pointed out that while the spores of a parasitic
fungus will germinate on the damp surface of the leaf of any plant, yet the
germ tubes will enter the tissues and infect only the particular kind of plant
on which the fungus is known to be parasitic. It is a legitimate inference
from this fact that the tissues of the particular host possess a special and
peculiar attraction for the parasite in question. This hypothesis has been
tested experimentally, and it has been found that under certain circumstances
positive chemotropism plays a prime part in inducing infection. Through the
use of certain chemical substances shown to attract certain fungi, an otherwise
immune host plant has been successfully infected. In other words, the entrance
of a parasitic fungus into the tissues of a healthy plant depends, at least
in some cases, on the presence of some positively chemotropic substance in
the cells of the host. It is stated that a saprophytic fungus can be gradually
educated to become an active parasite for a certain plant by introducing a
positively chemotropic substance into the tissues of that plant. It is possible
also by similar means to induce a specific parasitic fungus to invade a new
host hitherto not attacked by this particular parasite. In this connection
it has been pointed out that a remarkable coincidence exists between the behavior
of fungus spores toward their host, and that of the pollen grains when placed
on the stigmas of flowers of their own and of other species. The marked positive
and negative chemotropic properties shown in the attraction of pollen tubes
are apparently closely related to the similar manifestations of the germ tubes
of parasitic fungi. The strict association of specific parasite and specific
host may, perhaps, be explained on a similar chemotropic basis.